Modulating system



Oct, 6, 1936; F A, C KS 2,056,167

' MODULATING SYSTEM Filed 004;. 29, 1954 23 v 3 in? qmi YINVENTOR. Pause 1416697 (EMA/5 Patented Oct. 6, 1936 UNITED STATES PATENT OFFICE 6 Claims.

This invention relates to modulating systems, and particularly to methods and apparatus for modulating oscillators of the type disclosed in copending application, Serial No. 587,230, filed January 18, 1932, and patented on January 7, 1936, No. 2,027,160, by Richard E. Furay.

The Furay type of oscillator is characterized by great inherent frequency stability, which renders it unnecessary to provide therefor extraneous stabilizing equipment such as oscillating crystals or magneto-striction apparatus. Among the objects of this invention are: To provide means and methods for modulating such oscillators without destroying this inherent stability; to provide a method and means of producing, in a single stage, a high percentage of modulation; and to provide a system of modulation wherein a high degree of modulation may be obtained with relatively small power in the modulating tubes or other modulating equipment; and, as the result of the equalities above set forth, to provide a modulating system wherein satisfactory performance characteristics from the point of view of frequency stability and percentage of modulation may be obtained at low cost.

My invention possesses numerous other objects and features of advantage, some of which, together with the foregoing, will be set forth in the following description of specific apparatus embodying and utilizing my novel method. It is therefore to be understood that my method is applicable to other apparatus, and that I do not limit myself, in any way, to the apparatus of the present application, as I may adopt various other apparatus embodiments, utilizing the method, within the scope of the appended claims.

Referring to the drawing:

Figure 1 is a circuit diagram showing the application of my system of modulation to a Furay oscillator.

Figure 2 is a circuit diagram showing the system in modified form.

Figure 3 is a graph showing static characteristic curves of the oscillating tube.

The type of oscillator to which this invention is applicable comprises a vacuum tube having the usual electron-emitting hot cathode and electronreceiving cold anode. Cooperating with these electrodes are two control electrodes, asymmetrically placed with respect to cathode and anode, and operated at a bias potential such that similar changes in potential upon the two control electrodes produce opposite efiects upon the electron stream between the cathode and the anode. In general, this requires that a positive bias be placed upon the control electrodes. When these electrodes are negative with respect to the cathode, small changes of potential upon them will produce like changes in the electron flow, 1. e., small positive variations of potential on either '5 electrode will increase the space current. With increased positive potential, however, a point is reached where an increase in positive potential of one electrode will decrease the anode current, v

while a similar change upon the other electrode 10 will increase that current. There is a material range of potentials over which these conditions obtain, and this is the normal range of opera tion of the oscillator. A frequency-controlling element, comprising, preferably, a parallel res- 15 onant circuit, is connected between the two control electrodes, and this circuit is coupled loosely to the anode circuit of the oscillating tube. Under these conditions a variation in output current will cause one of the control electrodes to swing 6 positive and the other to swing negative, and since the control electrodes have opposite effects upon the output current, these opposite swings produce accumulative effects upon the output current flow, causing further change in electrode potential, 5 and serving to generate self-sustaining oscillations.

The intensity of oscillation of the tube is largely governed by the range over which the control electrodes have opposite efiects. Outside of this 30 range the opposite swings of potential of the two electrodes will nullify each other and produce little or no efiect upon the output current.

The modulating system of my invention is predicated upon the use ofthis type of oscillator. Considered broadly, it comprises applying potential variations at modulating frequency to both of the control electrodes in the same sense. The range of oscillating potential variation over which the control electrodes exert opposite effects is thereby varied, and with it the intensity of oscillation. It is therefore possible to effect complete modulation of the oscillating output of the tube.

This will be clearer from a consideration of the operation of the circuit as shown in Figure l. The oscillator tube 1 is of the Furay type, comprising the heated cathode Z, anode 3, and the'two control electrodes comprising a grid 5 and a reflector electrode 6 respectively.

Static characteristic curves of this tube are shown in Figure 3. The two curves of this figure show the anode current obtained when the potential of one of the control electrodes is varied whilethe other is maintained at zero or cathode 55 potential. It will be seen that for the entire range of potentials below those indicated by the value A, the two electrodes have like effects upon the output current. For potentials greater than this, positive swings of the grid increase the anode current, while like swings of reflector potential decrease this current. If, now, other curves be drawn, each representing the anode current variations when the potential of one of the control electrodes is maintained at some value other than zero, while the other is varied over a wide range of potential, families of curves are produced, each having its maximum at about the same positive potential as that shown for zero potential of the other control electrode, and approximately parallel to the curves of the figure. In order to obtain the detailed operation of the device, it is, of course, necessary to draw such families of curves, but the construction is somewhat confusing and is therefore omitted here. It should be suflicient to point out that maximum oscillation is secured with the two electrodes biased at a positive value equal to that indicated by the abscissa B, and that biasing potentials greater than C or less than A are operative practically to stop oscillation.

' The grid 5 and reflector 6 are connected to opposite ends of a parallel resonant circuit comprising an inductor l bridged by a variable condenser 8, this circuit being tuned to the desired frequency of oscillation and comprising the frequency-controlling element of the system. A second parallel resonant circuit comprising an inductor l0 and variable condenser H is connected between the anode 3 and the positive end .of a suitable source l2 of plate current, whose negative end is connected to the cathode. The details of the cathode heater circuit are omitted in the drawing, as they are conventional in type and can be readily supplied by those skilled in the art.

Any of the conventional forms of coupling between the control electrode and anode circuits may be used. The Furay application above mentioned describes several of these methods. In the circuits here shown, capacity coupling between the grid 5 and anode 3 is relied upon, the tap l3 being connected to the neutral point of the frequency-control circuit and connecting through a condenser l5, of relatively small capacitance, to the low potential side of the anode resonant circuit.

As in the case described in the Furay application, however, the bias for the control electrode is supplied through a resistor from the anode source, this resistor, however, being in the form of a conventional three-electrode vacuum tube l6 having its cathode or filament I! connected to the control electrode circuit, while the anode l8 connects directly to the plate supply I2. Variations in potential of the grid 20 of this tube will vary its impedance, and hence the potential drop therethrough and the mean potential of the two control electrodes 5 and 6. By proper choice of the normal potential of this grid, the points on the operating characteristics at which the two control electrodes operate may be varied at will.

The modulating frequency impulses are applied between the cathode and grid of the tube [6. In the present example this is accomplished by connecting the secondary coil 2| of an audiofrequency transformer between cathode and grid, while the modulating frequency impulses are applied to the primary 22 of this transformer, e. g., by the microphone 23 and battery 25.

As has been indicated above, when the potential of either of the control electrodes is varied alone, that of the other being held constant, the peak or maximum of the plate current occurs at approximately the same voltage, and the curve is of approximately the same form, differing primarily in magnitude. Thus the maximum current obtainable by varying the potential of the reflector 6 will occur substantially along the abscissa value A, while if the grid 5 be varied in potential the value for maximum plate current will occur at the abscissa C. Maximum output of oscillating current will therefore be obtained when the mean potential of the tube control electrode is at the value B, since this permits the maximum swing of both electrode potentials in opposite direction before the point is reached where the effect of either one or the other of these potentials is reversed, and their efiects tend to cancel out.

Thus, if the mean potential of the two control electrodes be established at the value A in Figure 3, a negative swing of the grid will be accompanied by a corresponding positive swing of the reflector. The net change in anode current will be a negligible value, and the oscillations will become of extremely small amplitude. A'similar effect will occur if the mean potential of the two control electrodes is established at the value C, but the power absorbed by the control circuit will be considerably larger. For optimum operation, therefore, the mean potential of the two control electrodes, as determined by the current flowing through the modulator or resistor triode IE, will be half way between the abscissas A and B.

When modulating frequency impulses are applied to the grid 20 of the modulating tube through the microphone 23 and the audiofrequency transformer, the mean potential of the two control electrodes will be varied about this mean value, and the amplitude of the oscillations will increase as the grid swings positive and decrease as the grid swings negative, producing substantially complete modulation as the bias potential of the two control electrodes 5 and 6 is varied between the limits A and B of Figure 3. Experiment has shown that practically 100% modulation may be procured with this arrangement where the load carrying capacity of the tube I6 is about 25% of that of the tube I. In view of the fact that throughout this range the oscillating circuit '|8 remains substantially the sole frequency-determining element in the entire apparatus, and that the frequency therefore remains substantially constant, this type of connection permits complete modulation together with a minimum of frequency variation and a minimum of installed apparatus. It is therefore particularly applicable to portable equipment.

In Figure 2 is shown a modification of this arrangement which differs from that of Figure 1 primarily in the method of applying the biasing and modulating potentials to the two control electrodes 5 and 6. The oscillating tube, frequency control circuit, anode circuit, and by-pass condenser are arranged in the same manner as in Figure 1, and are designated by similar reference characters. In place of the modulating tube, however, the bias is applied to the control electrodes through a variable resistor 30, connected to the positive end of the anode source I2. In series with this resistor is the secondary 3| of a transformer whose primary 32 is supplied by a microphone 33 and battery 35.

The operating range of the oscillator is selected by adjusting the value of the resistor 30 (the resistance of the transformer secondary usually being negligible in comparison With the rheostat resistance), and the potential variations induced by the voice current are superposed upon the mean bias thus supplied. The ultimate effect is substantially identical with that achieved by the arrangement of Figure 1, the modification of Figure 2 being, however, better adapted for use with very small power where a minimum of equipment is indicated.

I claim:

1. In combination with an oscillator having a pair of asymmetrically positioned control electrodes, a tuned circuit therebetween and a bias potential source for said electrodes whereby opposite variation of potentials on the control electrodes cumulatively affects the electron flow to produce oscillation, and modulating means comprising means for simultaneously varying the bias potential of both of said control electrodes in like sense at a modulating frequency to vary the amplitude of said oscillations.

2. In combination with an oscillator having a pair of asymmetrically positioned control elec trodes, a tuned circuit therebetween and a bias potential source for said electrodes whereby opposite variation of potentials on the control electrodes cumulatively affects the electron flow to produce oscillation, said bias potential source including means for applying a positive biasing potential to both said control electrodes, and means for varying said biasing potential at a modulating frequency, whereby the amplitude of said oscillations is varied at said frequency by variation of the range wherein opposite potential changes of said control electrodes have like effects.

3. In combination with an oscillator having a pair of asymmetrically positioned control elec' trodes, a tuned circuit therebetween and a bias potential source for said electrodes whereby opposite variation of potentials on the control electrodes cumulatively aiiects the electron flow to produce oscillation, said biasing source comprising a circuit connected to apply a positive potential to said control electrodes and including an impedance, and means for varying said impedance at a modulating frequency.

4. In combination with an oscillator having a pair of asymmetrically positioned control electrodes, a tuned circuit therebetween and a bias potential source for said electrodes whereby opposite variation of potentials on the control electrodes cumulatively affects the electron flow to produce oscillation, said biasing source comprising a circuit connected to apply a positive potential to said control electrodes, and means for varying at a modulating frequency the current flowing in said circuit.

5. In combination with an oscillator having a pair of asymmetrically positioned control electrodes, a tuned circuit therebetween and a bias potential source for said electrodes whereby opposite variation of potentials on the control electrodes cumulatively affects the electron flow to produce oscillation, said biasing source comprising a circuit connected to apply a positive potential to said control electrodes, a vacuum tube included in said circuit having a cathode connected to said oscillator control electrodes and itself provided with a control electrode, and means for applying potential variations of modulating frequency to said last mentioned control electrode.

6. In combination with an oscillator having a pair of asymmetrically positioned control electrodes, a tuned circuit therebetween and a bias potential source for said electrodes whereby opposite variation of potentials on the control electrodes cumulatively affects the electron flow to produce oscillation, said biasing source comprising a circuit connected to apply a positive potential to said control electrodes, a transformer having a secondary winding included in said circuit and a primary winding, and means for causing current variations of modulating frequency in said primary winding, whereby the effective impedance of said circuit is varied at modulating frequency.

FULLER ALBERT CROOKS. 

